News the deal had gone through came via the Blues' official website, with the Italy international signing a four-year contract with his new club.
On his move to west London, he said: "It is a dream for me to join Chelsea and play in the Premier League. I am very excited to be here and start working with the team. I hope I can show my quality to the coach, my team-mates and the supporters."
According to Sky Sports News, the Premier League champions have parted with around £23 million to get the wing-back in.
Zappacosta is set to provide competition on the right flank at Stamford Bridge. That's after the Blues missed out on signing Alex Oxlade-Chamberlain from Arsenal; he moved to Liverpool on Thursday despite the Blues agreeing a fee with Arsenal, per BBC Sport.
Chelsea manager Antonio Conte is clearly keen to add cover at the wing-back position, with Victor Moses lacking serious competition on the right side. In Zappacosta, he has a footballer capable of doing a similar job.
ESPN FC's David Amoyal took a look at what the 25-year-old will bring to Conte's side:
Meanwhile, as noted by OptaPaolo, the Blues' latest recruit loves to raid forward and get involved in attacking play:
After impressing for the Granata, Zappacosta earned a call-up to the Italy national side, where he worked under Conte. Evidently, the defender made an impression, and he will be desperate to impress in the Premier League after this lucrative switch.